Thermal switch



July 16, 1963 1.. w. cooK 3,098,137

THERMAL SWITCH Filed Oct. 10, 1960 2 Sheets-Sheet 1 I v at,

L. W. COOK THERMAL SWITCH July 16, 1-963 2 Sheets-Sheet 2 Filed Oct. 10, 1960 I I m w m, m. M/ H E H WC Mr m m z 1 Z 1 M M F l 55 H WW m ll m I ::I M M M ww wflw MW WW y 'the conventional glow switch are United States Patent 3,tl9$,l37 THERMAL SWITCH Leonard W. Cook, Warwick, RR, assignor to General Electric Company, a corporation of New York Filed Oct. 10, 1960, Ser. No.

12 Claims. (Cl. 2tltl-ll3.5)

This invention relates to a thermal switch; more particularly,

the invention relates to a novel thermal switch of the glow type used particularly in starters for fluorescent lamps.

Fluorescent lamps and other gaseous discharge devices require a high starting voltage but,

can operate on a much lower voltage. Normally,

lamps are designed for operation on conv once started,

fluoresentional residential voltages but these voltages are insui'ficient to start the lamps. the circuit with the fluorescent lamp an in'du or ballast and a glow starter. switch comprises a gas-filled tube in which Starting voltage is provided by including in ction element conventionally, the glow there is a bimetal contact strip spaced slightly from another contact strip, the two metals forming posed so that when heat is applied to the it will close the gap between contact strips. arrangement with the fluorescent the lamp-operating switch is closed, a

the bimetal strip being disbimetal strip, The circuit lamp is such that, when glow discharge occurs between the two contact strips of the switch and this heats the bimetal element which causes it to be deflected into contact with the contact strip.

As soon as the two contact strips come into contact with each other, a relatively large current flows from the power source through the starter electrodes and thence lamp filaments, to heat act of starting.

through the them in preparation for the final Also, conduction across the space between the two strips ceases and the bimetal strip begins to cool. As it cools, it breaks contact contact strip and this induces a the induction element which strikes an arc electrodes of the fluorescent lamp. It is the terruption of this relatively large preheat with its companion high voltage surge through between the prompt incurrent by the glow switch electrodes, typically about 0.5 amp. in a 40-watt lamp and 2.0 amp. in an 85-watt lamp, that develops the high voltage surge that starts the fluorescent lamp. Once the arc is struck, takes place within the lamp out of the circuit. However, it conduction through the contact gap conduction thereafter and the glow switch remains the arc fails to strike, in the glow switch will again take place and the cycle will be repeated. To prevent an indefinite repetition of this cycle on a faulty lamp, glow switch starters have been second bimetal element designed having a which on repeated starting cycles is slowly heated. This breaks the starter circuit by opening an external element and holds it open until such time present time two lead wires sealed into pieces of generally parallel both of the lead wires within the envelope, a nonoxidizing gas atmosphere within the envelope, and a metallic c oating on the bimetal or lead wires to promote ionization of the gas in the envelope.

problems best described by quoting No. 2,267,098, page 1, line 15:

The main problem in assembling t switches was the production of parts to clo The component parts which compose subject to processing from the Hays Patent hese (glow) se tolerances,

inspection of the parts and careful adjustment during assembly Bending the wires resulted in a great deal of shrinkage by breaking welds and cracking glass. During the exhaust (heating) another proportio n of switches "ice were lost because of roughness of contact wires causing locked contacts and inaccurate spacings.

or from Peters Patent No. 2,329,134, page 1, line 29:

. The heating of the switch parts incident to the (glass) sealing operation flexes the bimetallic electrode which frequently presses against the stationary electrode with such force that it becomes distorted in shape. On cooling the spacing of the electrodes is greater than the original spacing thereof and the breakdown voltage of the completed switch is higher than that desired.

or Ladig Patent No. 2,259,111, page 1, line 30:

The purpose of my invention is to provide a gaseous discharge thermal relay that will not be deformed during heat treatment and exhaust common to such devices.

The other principal problems involved in the manufacture of conventional glow switches have been concerned with (l) the ionizable gas used; (2) the pressure of the gas; (3) the dark ellect; and (4) the spacing and surface material of the electrodes.

For optimum functioning as a fluorescent lamp starter, certain undesirable compromises have heretofore been made. For example, it has been considered necessary for initiation of the glow discharge by the relatively low voltages conventionally available, that the electrode spacing be critically determined by reference to a relatively narrow range of optimum operation which was also aitected by gas pressure. However, the gas pressure was subject to conflicting requirements, since a relatively high gas pressure was desirable to achieve initial high current flow for rapid heating and operation of the bimetal element, yet a much lower gas pressure was desirable so that the inductive lamp starting surge would not be dissipated too much in the starter. As a result, relatively large electrode spacings were considered necessary. This in turn raised another problem, due to the so-called dark effect, wherein supplying an additional element, such as a radioactive gas, or aluminum paint, to the glow tube structure was required to insure reliable starting. In spite of these compromises, the starting of fluorescent lamps by prior art starters was by no means as rapid as is desirable, even by consumption of a relatively large portion of the inductive lamp starting surge.

Accordingly, it is a major object of the present invention to provide a novel thermal glow switch of unique construction and one much less subject to the conventional design compromises wherein, for example, rapid starting action is achieved only at the expense of available inductive striking voltage for the lamp, means must be provided for overcoming dark effect, electrode structures must be over-designed and over-built only for the purpose of surviving the high temperature manufacturing process, and cr'tical electrode spacing must be achieved.

It is another object of the invention to provide a novel thermal glow switch structure wherein the switch itself functions as a heating element as is particularly useful in resettable fluorescent lamp starters, either manual or auto matic.

Another of the objects of the present invention is to pro.- vide a glow switch which is less costly to manufacture than the conventional glass envelope glow switch.

Another object of the present invention is to provide a glow switch in which the spacing of the electrodes can be eiiected readily.

Another object of the invention is to provide a glow switch having a metal envelope or housing.

Another object of the invention is to provide a glow switch tree of radioactive or other auxiliary materials.

Another object of the invention is to provide a glow switch which is much smaller in size than the conventional glass-envelope glow switch.

The above and other objects of this invention will be understood from the following specification and accompanying drawing wherein FIG. 1 is a sectional view of a glow switch constructed in accordance with this invention;

FIG. 2 is an exploded perspective view of the elements entering into the structure illustrated in FIG. 1;

FIG. 3 is an enlarged broken section of the elements shown in the upper portion of FIG. 1 illustrating the preferred shape of the tip of the bimetal element;

FIG. 4 is an enlarged broken section of the upper portion of FIG. 1 illustrating the movement of the bimetal element during operation of the switch;

FIG. 5 is a perspective view of a fluorescent lamp starter using the glow switch of this invention;

FIG. 6 is a perspective view of an automatic reset starter;

FIG. 7 is :an exploded perspective view of a manual reset fluorescent starter incorporating in its structure the glow switch of the present invention; and

FIG. 8 is a graph showing striking voltage vs. electrode spacing and gas pressure for a representative number of gases.

Briefly stated, in accordance with its preferred embodiment, the present invention is directed to a hollow gastight metal housing, a terminal rod extending through an end of said housing, the terminal rod being insulatingly sealed from the housing, and an elongated bimetal element mounted on the rod on the interior of the housing closely adjacent to but spaced preferably from the end surface thereof, the bimetal element being deformable into contact preferably with the side of the cylindrical housing when subjected to a preselected temperature.

As shown in FIGS. 1 and 2, the preferred glow switch of this invention consists of a metal shell or housing 10 which is conveniently in the form of a hollow cylinder closed at one end to form a generally flat end surface 16 perpendicular to the cylinder axis and having a flanged portion 11 at the other end. An inexpensive material such as steel is entirely satisfactory for the housing 10' although more expensive materials such as brass or copper may also be used. A steel terminal rod 13 is positioned in a powdered glass preform 14 which is in turn positioned in a steel eyelet or ring 12. Initially, this assembly is positioned in a holding jig and then heated in a furnace at 1800 F. for one minute, a process which melts the glass and fuses it to the steel rod 13 and eyelet 12. to make a vacuum-tight electrically insulated seal which is very durable. Preferably, the eyelet 12 is composed of metal thick enough to compress the glass preform 14. With this structure the coefficients of expansion of the glass and metal are purposely mismatched to provide a compression bond in addition to the chemical bond between the two.

After the terminal-rod-steel-eyelet assembly has been fabricated, a bimetal strip 15 is welded to the terminal rod 13. The step of welding the bimetal strip 15 to the terminal rod 13 may be taken with a very long strip of bimetal which is cut to the proper length after the welding step is completed. Such bimetal may be, for example, of iron or stainless steel with brass with a precoat of zinc, aluminum, nickel or the like, or coated after assembly as by a getter device with barium, lanthanum, thorium, or the like.

The assembly formed by the eyelet 12, terminal rod 13, glass preform 14, and bimetal strip 15 is positioned in the housing 10 where the bimetal is held centrally by the cooperating dimensions of the flange 11 and eyelet 12. The accurate depth of the housing 10 to its surface 16 and accurate length of the bimetal strip 15 establish precise spacing between these two elements. Furthermore, any bending of strip 15, since such will be mainly in a direction along plane 16, will have negligible effect on the spacing. This is important in establishing a predetermined spacing as will hereinafter appear. The components are placed in the lower electrode of a resistance welding machine (not shown) and the upper electrode (not shown) is closed.

A significant improvement brought about in the manufacture of the glow switch of this invention is that at no time during the manufacturing process need the bimetal strip 1 5 be heated significantly above room temperature. Consequently, the position and the spacings of the bimetal strip with reference to the housing 16 are not disturbed by the assembly operations. Too, any bending of the strip little affects electrode spacing, the gap being in a direction along the bimetal strip and between its free end 15a and a surface 16 perpendicular to the long dimension of the strip. Thus, deflection of the strip end simply displaces it in a direction along the surface 1 6, without much effect on the gap, and this occurs during bimetal operation during which the strip end 15:: contacts the side of the housing ltia, as shown in dotted lines in FIGS. 1 and 4. As a result of the ease of obtaining proper dimensional relationships, the bimetal strip 15 can be made thin and narrow and, thus, more quickly responsive to the heat of the ionized gas, as is especially important when used as a fluorescent starter. Also, by the novel utilization of a surrounding outer electrode as a housing, the entire element will function as a heating element for operating thermal elements adjacent thereto as will be hereinafter described in more detail.

Another advantage of the glow switch of the invention is its provision of two electrode spacings, the first, as discussed above and hereafter as electrode spacing having to do with the glow current gap, and the second, hereafter called bimetal spacing having todo with the gap in the direction of movement of the bimetal element. Thus, these separate gaps may be made optimum for the particular function, so that if, for example, a large bimetal spacing is desired as is hereinafter discussed in more detail, the electrode spacing need not be controlled thereby but may be optimum, say much narrower. With this aspect of the invention in mind, other arrangements of the internal elements within the metal housing may be used in substitution for that of the preferred structure. For example, the terminal rod could be extended nearly the length of the housing, with the electrode gap between the end of said rod and the end of the housing, and with the bimetal strip extending from adjacent the upper end of the rod toward its base so that upon its deflection its free end would contact the housing adjacent the seal end thereof. Or the bimetal might well be in the form of a U with one arm thereof welded at the top or bottom of a long terminal rod and the other arm free to move into contact with the metal housing as might be desirable to improve temperature sensitivity without increasing the housing dimensions. In either of these structures, the electrode spacing is not between the bimetal strip and housing, but between the rod and housing and, to an extent even greater than in the preferred structure, has an electrode spacing independent of the bimetal spacing.

In addition to the above described advantages arising directly from the novel metallic housing structure as described above, I have also found that the novel utilization of a much narrower electrode spacing than has heretofore been thought practical results in improved starter switch operation without the compromises heretofore thought necessary. More specifically, electrode spacing has been heretofore defined within a relatively narrow range because of the characteristics of the ionizing gas within the glow structure. As will be seen in FIG. 8, the curves of striking voltage vs. the product of electrode spacing and gas pressure of such useful neon, and helium show a minimum striking voltage of the order of l-10 mm. Hg, times cm. With the usual commercially available voltages of about -120 volts, operation within such region has heretofore been thought to be essential. As a result, electrode spacings of about 0.030 inch with 40-50 mm. Hg pressure have been used but with the detriment of decreased inductive voltage available for lamp starting because of the high gas presgases as hydrogen, argon,

, that it is no longer operative.

su-re necessary for a high enough initial current llow for rapid glow switch operation.

In accordance with the principles of the present invention, however, much narrower electrode spacings may be and preferably are used, say about 0.0O 2-OkQ-l5 inch, and much lower gas pressures of the onder of 20 mm. Hg pressure may be used so that a higher inductive voltage is available for lamp starting. Surprisingly, such close spacings do not decrease the initial current flow between the electrodes or make it operable only at a higher voltage, as might be predicted from the showing of the graph of FIG. 8. On the contrary, I have found that the initial current flow with such spacings is usually substantially greater than with conventional electrode spacings, and accomplishes more rapid starting than with heretofore known structures. The reason for this unexpected desirable result is not entirely clear to me, but it is believed to be due to a spark initiation of glow current flow between the electrodes, rather than the usual ionization.

Furthermore, by utilizing such electrode spacings which are within the mean free electron path of the conventional materials available as bimetallic element coatings as set forth above, the so-called dark effect is entirely eliminated, as is the necessity of supplying an additional element, such as a radioactive gas, for the purpose of overcoming the dark effect, which is assumed to result from a complete absence of ions within the switch enclosure after the switch has been idle in the dark for about 24 hours. In the absence of stray ions in the fill gas, the voltage necessary to cause the switch to function increases beyond usable limits. In conventional glassenvelope switches, this deficiency has been overcome by introducing a small amount of radioactive gas (Krypton 85) into the argon or other fill gas which is used and the radioactive material continually supplies ions to keep the breakdown voltage of the switch at a low operating level. Another expedient has been to apply aluminum paint across the terminal leads within the glass enclosure. Even though the resistance of the aluminum coating is very high, some current flows through the metallic coating when operating voltages are applied and this current produces suflicient ions to trigger the gas flow after the switch has been idle and in the dark for a prolonged period.

The above discussion should not be taken to mean, nevertheless, that wider electrode spacings may not be used in the novel meta-l housing structure characteristic of the invention, as wider gaps are usable but suffer from the deficiencies of the prior art, as set forth above.

FIG. 5 illustrates an improved plain fluorescent lamp starter using the glow switch of this invention. A conventional starter base 20 has a pair of protruding terminals (out of sight in FIG. 5) connected to tabs 21a across which a glow switch of this invention is electrically connected. A condenser 30 is connected across the tabs 21a in parallel with the glow switch It) in order to provide radio interference suppression and reduce peak voltage across the glow switch 10.

FIG. 6 illustrates an automatic reset starter which has the same elements as the plain starter of FIG. 5 plus a resistor 31 and shunting bimetal element 32 in the glow switch circuit. Repeated cycling of the glow switch due to failure of its associated fluorescent lamp to start generates suihcient heat to deflect the bimetal 32 so that it no longer shunts the resistor 31. When this occurs, the resistor 31 so limits the current to the glow switch 10 However, because of the through the glow switch and resistor 31, the switch 1t) remains hot enough to maintain the deflection of the bimetal 32 until the wall switch is shifted to open the circuit. The bimetal 32 will then reassume the position illustrated in FIG. 6 until the circuit through the wall switch is again closed at which time the cycle described above will be repeated.

continuing circuit FIG. 7 illustrates an improved manual reset fluorescent lamp starter utilizing the glow switch of the present invention. Aside from the glow switch and the improvement brought about by its use, the parts are similar in function and appearance to the corresponding parts in the starter of Young et a1. Patent No. 2,417,724. A starter base 20 with a pair of protruding terminals 21 and tabs 21a has an insulating supporting plate 22 with a central aperture through which the glow switch housing It? extends mounted on tabs 21a. A latching bimetal strip 23 mounted upon the eyelet 12 has its free end 24 curved to form a latch for positioning a spring 25. When the starter is positioned in the housing 26 having an insulating paper liner 27, a reset button 28 may be manually pressed to position the spring 25 into latching engagement with the bimetal strip 23. In the event the starter goes through a number of cycles in an attempt to start a defective lamp, the glow switch becomes hot and a sufiicient portion of this heat is transmitted to the bimetal element 23 so that its latching end 24 swings out of position to release the spring wire 25. This disables the operating circuit for the starter until such time as the reset button 23 is pressed to restore the spring wire 25 to the position illustrated in FIG. 3. The advantage of the present structure over the structure illustrated in Patent No. 2,417,724 is in the elimination of a separate heating coil which must be used when a glass glow switch is in the circuit. The metal glow switch of the present invention is so much a better heat conductor than a glassenvelope switch that the glow switch quickly heats up on repeated cycling to deform the bimetal element 22 to point of releasing the spring wire 25.

In addition, in the novel structure of the present invention with its independent electrode spacing and bimetal spacing, the bimetal spacing may be adjusted to control the available heat without upsetting the optimum electrode spacing as has been set forth in some detail above. More specifically, in order to produce a high bimetal closing temperature for actuating an auxiliary device which requires a relatively high heat, say 200 C., the bimetal spacing may be about 0.050 inch, far higher than the optimum electrode spacings of the present invention. Thus, it is unnecessary, as in heretofore known structures, to compromise on a single gap spacing which cannot be correct in many instances for both glow opera tion and bimetal operation to produce a desired temperature and operating rate.

While the present invention is directed to specific structures, it is obvious that there may be variations which still retain the features thereof. Accordingly, the present invention should be limited in scope only as may be necessitated by the scope of the appended claims.

What I claim as new and desire to secure by Letters Patent of the United States is:

l. A thermal switch comprising a hollow gas-tight metal housing, a terminal rod extending through one end of said housing, said terminal rod being insulatingly sealed from said housing, and an elongated bimetal element mounted on said rod on the interior of said housing, said bimetal element having an end portion spaced from the wall of said housing to provide a glow discharge between said end portion and said housing When said thermal switch is part of an electrical circuit and being deformable into contact with said housing when subjected to a preselected temperature, the glow discharge and contact areas of said housing being in different portions thereof.

2. A thermal switch as claimed in claim 1 wherein the interior of the cylindrical housing contains a non-oxidizing gas.

3. A thermal switch comprising a hollow gas-tight cylindrical metal housing, an axially positioned terminal rod extending through one end of said housing, said terminal rod being insulated from said housing by means of a glass seal which positions said terminal rod in said housing, a bimetal element mounted on said rod on the interior of said housing, said bimetal element having an end portion normally out-of-contact with said housing and spaced from an end wall of said housing to provide a glow discharge between said end portion and said end wall when said thermal switch is part of an electrical circuit and being deformable into contact with a side wall of said housing on an increase in temperature, and a non-oxidizing gas within said housing.

4. A thermal switch as claimed in claim 3 wherein the normal gap between bimetal and housing is between 0.002 and 0.015 inch and the pressure of the nonoxidizing gas is about 20 mm. mercury 5. In a fluorescent lamp starter having manually reset circuit breaking means, an insulating supporting plate defining an aperture, a thermal switch as claimed in claim 3 positioned in said aperture, a bimetal contact strip mounted at one end on the flange of the thermal switch eyelet member and having a latch at the other end, and a resilient contact member movable into latching engagement with said latch, the latch serving to maintain contact through said resilient contact member when said contact member is latched thereon.

6. A thermal switch comprising a hollow cylindrical metal housing closed at one end and flanged at the other, a flanged metal eyelet member, the flanges of said eyelet member and said housing being welded in gas-tight engagement, a terminal rod extending through said eyelet, a glass seal positioning said rod in said eyelet member and insulating said rod from said eyelet member, and a bimetal member mounted on said rod on the interior of said housing, said bimetal member having an end portion with a normal position out-of-contact with said housing and in glow discharge relationship with a particular portion thereof and being deformable by a temperature change into contact with a dilferent portion of said housing.

7. A fluorescent lamp starter comprising a pair of terminals mounted on an insulating base, and a condenser and glow switch connected in parallel across said terminals, the glow switch consisting of a hollow gas-tight metal housing, a terminal rod extending through one end of said housing, said terminal rod being insulatingly sealed from said housing, and an elongated bimetal element mounted on said rod on the interior of said housing, said bimetal element being in glow discharge relationship with one wall of said housing and being deformable into contact with another wall of said housing when subjected to a preselected temperature.

8. A thermal switch comprising a hollow gas-tight metal housing, an electrode assembly including a terminal rod and an elongated bimetal element mounted thereon, said terminal rod extending through said housing and being insulatingly sealed therefrom with said elongated bimetal element mounted on said rod in the interior of said housing, said electrode assembly being spaced from said housing to provide an electrode spacing for initial current flow between said electrode assembly and said housing and a bimetal spacing between said bimetal element and said housing independent of said electrode spacing, and a non-oxidizing gas within said housing.

9. A thermal switch comprising a hollow gas-tight metal housing having a side surface and an end surface, a terminal rod extending through said housing, said terminal rod being insulatingly sealed from said housing, an elongated bimetal element mounted on said rod on the interior of said housing and extending toward said end surface thereof providing an electrode spacing for initial current flow between the bimetal element and the end surface of said housing and a bimetal spacing between the bimetal element and the side surface of said housing independent of said electrode spacing, and a non-oxidizing gas within said housing.

10. A thermal switch as claimed in claim 9 wherein the electrode spacing is between 0.002 and 0.015 inch and the open bimetal spacing substantially greater than said electrode spacing.

11. A thermal switch as claimed in claim 10 wherein said open bimetal spacing is about 0.050 inch.

12. A thermal switch comprising a hollow gas-tight metal housing, a terminal rod extending through one end of said housing spaced in glow discharge relation with the opposite end of said housing, said terminal rod being insulatingly sealed from said housing, and an elongated bimetal element mounted on said rod on the interior of said housing near the free end thereof and extending toward the sealed portion of said rod, the free end of said bimetal element being deformable into contact with said housing when a glow discharge Within said housing heats said bimetal to deforming temperature, the glow discharge and bimetal contact areas being in different portions of said housing.

References Cited in the file of this patent UNITED STATES PATENTS 2,247,872 Chirelstein July 1, 1941 2,343,262 McCarthy Mar. 7, 1944 2,355,476 Smith et al. Aug. 8, 1944 2,392,969 Bickley Jan. 15, 1946 2,417,724 Young et al. Mar. 18, 1947 2,497,397 Dales Feb. 14, 1950 2,773,962 Perst Dec. 11, 1956 2,855,485 Webking Oct. 7, 1958 FOREIGN PATENTS 1,102,061 France May 4, 1955 

12. A THERMAL SWITCH COMPRISING A HOLLOW GAS-TIGHT METAL HOUSING, A TERMINAL ROD EXTENDING THROUGH ONE END OF SAID HOUSING SPACED IN GLOW DISCHARGE RELATION WITH THE OPPOSITE END OF SAID HOUSING, SAID TERMINAL ROD BEING INSULATINGLY SEALED FROM SAID HOUSING, AND AN ELONGATED BIMETAL ELEMENT MOUNTED ON SAID ROD ON THE INTERIOR OF SAID HOUSING NEAR THE FREE END THEREOF AND EXTENDING TOWARD THE SEALED PORTION OF SAID ROD, THE FREE END OF SAID BIMETAL ELEMENT BEING DEFORMABLE INTO CONTACT WITH SAID HOUSING WHEN A GLOW DISCHARGE WITHIN SAID HOUSING HEATS SAID BIMETAL TO DEFORMING TEMPERATURE, THE GLOW DISCHARGE AND BIMETAL CONTACT AREAS BEING IN DIFFERENT PORTIONS OF SAID HOUSING. 